Networking is progressing in various technical fields lately due to the advancement of communication technologies, and numerous apparatuses within a building are connected to networks. For example, various apparatuses including illumination, air conditioning, measurement and crime prevention apparatuses are installed in such buildings as apartment houses, government office building, concert halls, commercial buildings, as office buildings and private homes, and these apparatuses are interconnected via predetermined transmission lines to be networked, so it is now possible for a supervisory control apparatus connected to a network to perform centralized supervision and/or control (perform at least one of centralized supervision and centralized control).
In a supervisory control system that performs centralized supervision and/or centralized control by networking a plurality of apparatuses like this, a communication protocol for supervisory control, for transmitting/receiving such data as command data and supervisory data, is used. Examples of the communication protocol for supervisory control are: communication protocols conforming to the RS485 standard, which is a typical communication protocol; LonWorks® (Local Operating Networks), which is an intelligent distributed control network technology developed by Echelon Corporation; and NMAST®, which is advocated by Panasonic Electric Works. Co., Ltd. NMAST® is characterized in that the wiring topologies are free, and pair lines can be used for the transmission lines.
Decoding is required in order to extract data from a communication signal transmitted over such a network. Examples of a decoding method are: a synchronous detection method wherein a signal having a predetermined phase is extracted (carrier wave reproduction) from a communication signal (receive signal) received from the network, and decoding is performed based on this extracted signal (reference phase signal); and a delay detection method wherein decoding is performed by comparing phases of the received waves of adjacent symbols (time slots), that is, by regarding a value of the data as “0” (or “1”) if the phase is the same as a signal of the previous symbol (reference signal), and regarding the value of the data as “1” (or “0”) if the phase is different from this signal. In the case of the delay detection method, which performs decoding as mentioned above, it is unnecessary to generate the reference phase signal by reproducing the carrier as in the case of the synchronous detection method.
An example of a circuit based on this delay detection method is a delay detector circuit disclosed in Patent Document 1. The delay detector circuit disclosed in Patent Document 1 comprises: a limiter amplifier that transforms an intermediate frequency signal of a received two-phase PSK modulated wave into a rectangular wave signal, and amplifies the rectangular wave; an edge detection unit which extracts only a rise edge of the rectangular wave signal; a sawtooth wave generator that generates a sawtooth wave signal having a cycle based on the frequency of the intermediate frequency signal; a first sample and hold circuit that samples and holds the sawtooth wave signal using the rise edge, and detects a phase of the receive signal as voltage; a second sample and hold circuit that samples and holds the sampled and held signal by a timing signal of which timing is delayed by one symbol; a subtraction circuit that detects a phase difference of adjacent two symbols by subtracting an output signal of one of the sample and hold circuits from that of the other sample and hold circuit; a timing reproduction circuit that generates a symbol timing signal synchronizing with the output signal of the subtraction circuit; and an identification circuit that identifies the output signal of the subtraction circuit, and outputs the reproduced data.
For the delay detector circuit, low cost and low power consumption are demanded in the same way as standard circuits.
Patent Document 1: Japanese Patent Application Laid-Open No. H5-183593